Lean burn, low emission gas turbine engine combustors of the type now being developed for future engine applications have a tendency, under certain operating conditions, to produce audible pressure fluctuations which can cause premature structural damage to the combustion chamber and other parts of the engine. These pressure fluctuations are audible as rumble which occurs as a result of the combustion process.
Pressure oscillations in gas turbine engine combustors can be damped by using damping devices such as Helmholtz resonators, preferably in flow communication with the interior of the combustion chamber or the gas flow region surrounding the combustion chamber.
The use of Helmholtz resonators has been proposed in a number of earlier published patents including for example U.S. Pat. No. 5,644,918 where a plurality of resonators are connected to the head end, that is to say the upstream end, of the flame tubes of an industrial gas turbine engine combustor. This type of arrangement is particularly suitable for industrial gas turbine engines where there is sufficient space at the head of the combustor to install such damping devices. The combustor in a ground based engine application can be made sufficiently strong to support the resonators and the vibration loads generated by the resonators in use. This arrangement is not practicable for use in aero engine applications where space, particularly in the axial direction of the engine, is more limited and component weight is a significant design consideration.
A different approach to combustion chamber damping is therefore required for aero engine applications where space is more limited and design constraints require that the resonators are supported with respect to the combustion chamber without adding appreciably to the weight of the combustion chamber itself.
One form of Helmholtz resonator that is particularly suitable for a combustion chamber for aero engine applications is described in EP 1,424,006A2. The arrangement provides at least one Helmholtz resonator having a resonator cavity and a neck in flow communication with the interior of the combustion chamber, the neck having at least one cooling hole extending through the wall thereof. The cooling hole directs a film of cooling air on the inner surface of the tube wall in the region of the combustor opening, the film protecting the tube from the effects of the high temperature combustion gasses entering and exiting the resonator neck during unstable combustor operations.
It has now been found that at certain operating conditions the resonator body can overheat despite the presence of a cooling flow through holes in the neck of the resonator. This arises due to the movement of hot gases into, and out of, the resonator neck when pressure oscillations inside the combustor are relatively high. In addition, it is thought the holes angled towards the combustion chamber can induce a flow which draws hot combustion gases into the neck and resonator body.
Within the combustor, as discussed above, space available is often insufficient to place a conventionally structured Helmholtz resonator, which has a resonator cavity and a neck that extends therefrom. The limited space may require such resonators to be located away from their optimum point, or to have shortened necks, or to have resonator cavities of a volume that is not optimum for the frequency oscillation to be damped.
Because of the size of the conventionally structured Helmholtz resonators they are typically mounted radially inwardly of the wall of the combustion chamber and are typically mounted to the inner casing to avoid loads being transmitted to the combustion chamber itself. This positioning places the resonators close to the engine shaft where there can be problems with windage that require the resonator to be mounted within an isolating enclosure that reduces the windage effects. It will be appreciated that the isolating enclosure adds cost and weight to an engine.
It is an object of the present invention to seek to provide an improved damper arrangement for a combustion chamber.